Buletinul Ştiinţific al Universităţii “Politehnica” din Timisoara, ROMÂNIA
Seria CHIMIE ŞI INGINERIA MEDIULUI
Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 54(68), 1, 2009
Electrochemical Synthesis of Pyrazine-2,3-Dicarboxylic Acid.
Studies Carried Out Using Cyclic Voltammetry on a Stainless Steel
Electrode
I. Ţăranu, I. Popa, A. Dragos, D. Buzatu, M. Dobrescu
National Institute of Research-Development for Electrochemistry and Condensed Matter Timişoara, România
300569, Dr. A.P Podeanu, 144, Pho: 0256-222.119, Fax: 0256-201.382, e-mail: ioan_taranu@yahoo.com, www.incemc.ro
Abstract: The studies show the regeneration process of KMnO4 on stainless stell electrode, comparative with the ones
obtained on platinum electrode. The studies were made through cyclic voltammetry at concentrations of support electrolyte
(KOH) of about 5 to 10%, at 25-40ºC temperature, KMnO4 concentration until 6·10-2 M and maximum concentration of
quinoxaline until 2.61·10-2 M, in electrolyte solution. It seems that on stainless steel electrode is not possible to do the
oxidation of Mn6+ to Mn7+ like on paltinum [1,2], nickel [3] or copper electrode [4]. However, a final decision will be
made after the effectuation of new investigations (preliminary synthesis).
Keywords: electrochemistry, cyclic voltammetry, quinoxaline, pirazine-2,3-dicarboxilic acid, stainless steel electrode
permanganate and quinoxaline (solutions dissolved into
KOH of the same concentration used in measurements – 5
and respectively 10%) was 1N and 0.5M respectively. The
working intervale used was 0÷+700 mV for both stainless
steel and platinum electrode. The speed of polarization was
100 mV/s. Every time, the third voltammogram recorded
was kept. All the potentials in this paper are reported to
ESC (if is not mentioned otherwise).
1. Introduction
Due to the simplicity in manufacturing and of the low
price, after the studies carried out using platinum [1,2],
nickel [3] and copper [4] electrodes, we focused on
studying the possibility of using stainless steel electrode in
the process of Mn7+ regeneration. The Mn7+ ions can be
used in the process of chemical oxidation of quinoxaline in
order to obtain pyrazin-2,3-dicarboxylic acid. Because one
of the most suggestive results were obtained by means of
platinum electrode, we compared all the results of the
stainless steel electrode with the results obtained using a
platinum electrode.
3. Results and discussions
Figure 1 presents the curves obtained by cyclic
voltammetry at 25ºC using a solution of KOH 5% for
different concentrations of KMnO4 on platinum (B),
respectively stainless steel (A). The curve 1 is obtained
using KOH 5% in the absence of Mn7+. In the given
interval no process takes place. The release of the oxygen
onto the surface of platinum takes place at a higher
potential than 700 mV and the same process on stainless
steel takes place over 600 mV.
The increase in Mn7+ concentration on platinum leads
to the modification of the curves, 3 peaks appear: an anodic
peak at 450 mV and two cathodic peaks at about 300 mV
and respectively 80 mV. On the platinum when the increase
of the Mn7+ concentration occurs in the electrolyte, the
modification of the curves is observed.
The intensity of the peaks increases with the increase
of Mn7+ concentration. In the same time the peaks shift
toward more negative values (the cathodic peaks),
respectively towards more positive values (the anodic
peaks). It is clear that on the electrode surface, in the
presence of manganese ions several processes take place.
From the aspect of the curves we can observe that the
anodic peak and the cathodic peak 1 correspond to a
reversible process, probably the one illustrated by the
reaction below:
2. Experimental
In order to carry out the studies we used the same cell
of electrolysis used before [1,2,3,4] equipped with three
electrodes – a platinum counter electrode, a reference
electrode, ESC, and the working electrodes used for
measurements were stainless steel electrode and
respectively the platinum electrode. We used a stainless
steel wire with a diameter of 0.025 cm2. The platinum
electrode was made up of a platinum wire with an exterior
surface of 0.2 cm2. The electrodes were cleaned before
each measurement by diving into diluted nitric acid, then
into a solution of oxalic acid in 20% sulfuric acid for about
10 minutes. The electrode cleaned this way was used for a
series of measurements. Sometimes – when the results
demanded – the process of cleaning was repeated.
The conditions of work were the same as those used
for the study of the platinum electrode [1,2].
The studies of cyclic voltammetry were carried out
using a potentiostat PGZ 301 DynamicsEIS ValtammetryRadiometry Copenhagen, equipped with VoltaMaster 4 as
an operating system. As a support electrolyte we used
solutions of 5% and 10% of KOH, at temperatures ranging
between 25 and 40ºC. The concentration of the potassium
Mn7+ + e-
41
↔ Mn6+
(1)
Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 54(68), 1, 2009
for the advanced dissociation of the potassium manganate
and permanganate ions, so the forming of the MnO2 is
totally unwanted.
The second cathodic peak corresponds to a more
advanced reduction of the Mn7+, probably leading to Mn4+.
Figure 1. T = 25 ºC, sol KOH 5%, concentration of KMnO4: 1 – 0,
2 – 2·10-2 M, 3 – 3,8·10-2 M, 4 – 6·10-2 M, A – stainless steel electrode,
B – platinum electrode.
Figure 2. T = 25 °C, sol. KOH 5%, concentration of KMnO4 6·10-2 M,
conc. of Q: 1 - 0, 2 – 1.18·10-2 M, 3 – 2.04·10-2 M, 4 – 2.61·10-2 M,
5 - 3.62·10-2 M, A – stainless steel electrode, B – platinum electrode.
In the absence of the organic substance, the source of
Mn6+ is the dissociation in basic environment of the Mn7+.
On the stainless steel electrode (curve A, figure 1) the
introduction of KMnO4 in the electrolyte, in concentrations
of maximum 6·10-2 M, does not lead to supplementary
peaks – so no other process takes place on the electrode
than those observed using the platinum electrode.
The oxygen formation slightly shift towards more
positive values. It seems that on the stainless steel electrode
the regeneration of Mn7+ does not take place.
In the presence of quinoxaline the curves presented in
figure 2 are obtained.
On platinum (B) the presence of quinoxaline in the
electrolyte causes the slight modification of the curves. The
peaks corresponding to the reversible process: Mn7+ + e ↔
Mn6+ are especially modified. The intensity of the peaks
increases with the concentration of Q and even shifts with
10 mV towards more negative values. From those observed
in figure 2 it seems that the adding of Q has an effect
similar to the effect caused by the increase of Mn6+
concentration – the increase of the anodic peak – increase
which can be motivated by the increase in the
concentration of Mn6+ in the electrolyte as a result of the
chemical reaction between Mn7+ and Q. The second
cathodic peak seams to decrease in intensity – effect
associated to the decrease of the reduction of Mn6+ to Mn4+,
process desirable to our process of Mn7+ regeneration. As
we know, Mn4+ cannot be turned to a superior oxidation
status (6+ or 7+), because it precipitates in the electrolyte
and can be considered lost for the process of Mn7+
regeneration. Moreover, its presence has a catalytic effect
As a comparison with platinum, on the stainless steel
electrode only a process of shifting the release of oxygen
towards more positive values takes place. This process is
not proportional with the quantity of Q. The variation is not
uniform and can be a result of the absorbance of Q onto the
electrode. Though less likely, the direct oxidation of Q on
the electrode could take place together with the oxygen
release. Anyway, what we want is the regeneration of Mn7+
from Mn6+ and this process is not to be seen on the curves.
If the experiments are carried out in solution of KOH
5% at 40ºC we obtain the curves presented in figure 3.
On the basic curve, in the absence of potassium
permanganate we cannot notice any electrode process in
the interval studied (curves 1). On the platinum electrode
(B) the adding of potassium permanganate leads once again
– as it did at 25ºC (figure 1B) – to the increase of the peaks
corresponding to the reversible process (1) and to the
accentuation of the cathodic peak corresponding to the
reduction of Mn4+. In the moment we add Mn7+ in the
system we can notice the presence of Mn6+ formed by
chemical reduction in basic medium. The peaks are better
defined than at 25ºC and the increase of temperature causes
the increase of the intensity for the peaks, which shows the
favorable effect over the electrode process of the increase
in temperature of the electrolyte.
At the same composition of the electrolyte, the
increase of the temperature has as effect the increase of the
intensity of the anodic peak with 30%, while the cathodic
peaks suffer a 30% respectively 20% increase.
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Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 54(68), 1, 2009
Figure 3. T = 40°C, sol. KOH 5%, concentration of KMnO4: 1 – 0,
2 – 2·10-2 M, 3 – 3.8·10-2 M, 4 - 6·10-2 M, A – stainless steel electrode,
B – platinum electrode.
Figure 4. T = 40 °C, sol. KOH 5%, concentration of KMnO4 6·10-2 M,
concentration of Q: 1 – 0 , 2 – 1.18·10-2 , 3 – 2.04·10-2, 4 – 2.61·10-2 M,
5 – 3.62·10-2 M, A – stainless steel electrode, B – platinum electrode.
If the studies are carried out on a stainless steel
electrode (A), the increase of the KMnO4 concentration
does not lead to the apparition of any peaks but only the
slight shift of the potential for the oxygen release to more
negative values. This time the depolarization is
proportional to the concentration of Mn7+. Not even at this
temperature be cannot obtain proof of the fact that the
regeneration of Mn7+ would be carried out on the stainless
steel electrode.
The presence of Q in the electrolyte is presented in
the curves from figure 4.
The presence of Q on the stainless steel electrode (A),
at this temperature does not modify the curves. We can
observe an accentuated polarization at the highest
concentration of Q, 3.62·10-2 M (figure 4A).
The studies carried out on the platinum electrode in
the presence of quinoxaline (B) clearly show the peaks of
the reversible process (1) and the peak caused by the
reduction of Mn4+. Unlike the situation at 25ºC, where the
presence of Q had as effect the increase of both the peaks
of the reversible process – and the anodic peak at about 480
mV and the cathodic peak at 210 mV – at 40ºC we can
observe the increase of the anodic peak, proportional with
the concentration of Q, but the cathodic peak decreases.
This decrease is proportional with the concentration of Q.
In the same time, the potential of the anodic peak shifts
towards more positive values with the concentration of Q,
while the cathodic potential remains practically constant.
The values of the anodic potentials shift toward more
positive values than at 25ºC and the cathodic shift towards
more negative values. This status can be explained by the
increase of Mn6+ as a result of the increase in the speed of
the reaction of Q with Mn7+ (with generation of Mn6+)
along with the increase of temperature, for the anodic peak.
The decrease of the cathodic peak can be associated
with the decrease of Mn7+ concentration. On one hand
Mn7+ is consumed by the Q introduced into the electrolyte
(with a higher speed at 40ºC) and on the other hand it
seems that the regeneration of Mn7+ has not a sufficiently
high speed.
This would lead to the extinction of Mn7+ from the
solution of electrolyte and finally to the end of the reaction.
We must then give a special attention to this aspect because
it seems that temperature has a significant effect towards
the process carried out at the electrode.
The increase of the KOH concentration at 10%, at a
temperature of 25ºC leads to the curves presented in fig. 5.
Neither in KOH 10% on stainless steel the desired
peaks do not appear, only a polarization process resulting
from the release of oxygen.This polarization is proportional
to the concentration of KMnO4 (A). We cannot notice any
modification of the curves obtained in the presence of
potassium permanganate. This leads to the conclusion that
there are no signs of the regeneration of Mn7+.
On platinum, the presence of potassium permanganate
causes the apparition of 4 peaks – two anodic and two
cathodic. We can notice the presence of the two peaks
characteristic to the reversible process (1) – the anodic
peak at 450 mV and the cathodic peak at 320 mV. The
peaks increase with the concentration of Mn7+, are better
defined than in KOH 5% and are shifted towards more
negative potentials the anodic one and towards more
positive potentials the cathodic one. We can observe thus a
closeness of the two peaks, a better definition of every peak
and even of the reversible process in comparison to the
reactions carried out in KOH 5%. The release of oxygen
shifts towards more negative values along the increase of
KMnO4 concentration.
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Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 54(68), 1, 2009
Figure 5. T = 25 °C, sol. KOH 10%, concentration of KMnO4: 1 – 0,
2 – 2·10-2 M, 3 – 3.8·10-2 M, 4 - 6·10-2 M, A – stainless steel electrode,
B – platinum electrode.
Figure 6. T = 25 °C, sol. KOH 10%, concentration of KMnO4 6·10-2 M,
concentration of Q: 1 – 0 , 2 – 1.18·10-2 , 3 – 2.04·10-2, 4 – 2.61·10-2 ,
5 - 3.62·10-2 M, A – stainless steel electrode, B – platinum electrode.
A similar evolution can be observed at the other two
peaks. The cathodic peak – present for KOH 5% - is with
about 1 mA/cm2 greater than for KOH 10%, is slightly
shifted towards more negative values and is probably
associated with the reduction of Mn4+. A special remark
must be done for the second anodic peak, at 120 mV.
Though small in intensity this peak is well defined,
especially at greater concentrations. This peak is not
present in KOH 5%. The process which can be associated
to this peak is probably the oxidation of Mn2+.
It is obvious that with the increase in KOH concentration
to 10% in the electrolyte the peaks associated to the
electrode processes increase in intensity and are better
defined than in KOH 5% for the platinum electrode while
for the stainless steel electrode no other peak can be
observed.
In the presence of Q in KOH 10% the next curves can be
obtained (figure 6):
In figure 6 A the curves are obtained using a stainless
steel electrode in the presence of Q, showing the same
situation as for the KOH 5%, a not uniform evolution but
with trends of polarization of the oxygen release. Still we
cannot notice any signs of transformation of the manganate
ions.
The studies carried out on a platinum electrode using an
important amount of quinoxaline in electrolyte, lead to the
results presented in figure 6 B. The increase of the Q
concentration has no other significant effects on the curves.
The anodic peak at 440 mV increases very easily, while the
associated cathodic peak decreases slightly. These
observations are suitable for the peaks around 100 mV. In
these conditions it seems that the processes associated to
these peaks are reversible.
In this case, the cathodic peak decreases in intensity with
the increase of Q concentration while the anodic peak has a
slight trend of increase. Anyway, the differences between
the curves are not very big yet the trend seems that
presented above. On the platinum electrode we can achieve
the regeneration of Mn7+ in KOH 10% at 25º.
The increase of KOH concentration in the support
electrolyte has as effect the accentuation of the peaks
observed and a better definition of the electrode processes
which leads to positive effects on the desired reaction of
Mn7+ regeneration.
The curves obtained at 40ºC, with KMnO4
supplementary added are shown in figure 7.
On the curves recorded for the stainless steel electrode
there is no process, except the release of oxygen. It is pretty
clear that on the stainless steel electrode there is no process
of oxidation of Mn6+ to Mn7+ so that not even at 40ºC in
KOH 10% the regeneration of Mn7+ used for the chemical
oxidation of Q resulting in pyrazine-2,3-dicarboxylic acid
is not possible.
On the platinum electrode, at 40ºC in KOH 10%, the
aspect of the curves is the one described above; we can
notice the same two anodic peaks – at about 440 mV and
one at about 110 mV, rather weak, and visible only at
greater concentrations – and two cathodic peaks at 310 and
respectively 40 mV. The processes associated to these
peaks are those mentioned. We should also mention that
the intensity of the cathodic peaks is greater at 25ºC in
KOH 10% demonstrating the favourable effect of the
temperature over the electrode reactions.
In presence of Q at 40ºC and in KOH 10% we can obtain
the curves presented in figure 8.
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Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 54(68), 1, 2009
-
-
-
the first anodic peak, from about 130 mV is
slightly shifted towards more positive values than
the situation without Q;
the anodic peak from 440 mV increases in
intensity with the concentration of Q – proving the
increase of Mn6+ with Q;
both cathodic peaks decrease in intensity with the
increase in Q;
the decrease is more visible for the cathodic peak
from 50 mV.
Along the increase of Q concentration we can observe
a polarization process of oxygen release, process happening
only when Q is added into the electrolyte. The gradual
adding of Mn7+ has as effect a depolarization of the oxygen
emanation.
4. Conclusions
From the studies carried out on the oxidation process
of Mn6+ to Mn7+ on the platinum and stainless steel
electrodes by cyclic voltammetry, for the regeneration of
the potassium permanganate which can be used for the
oxidation of quinoxaline to pyrazine-2,3-dicarboxylic acid
we can sum up that the process takes place very well on a
platinum electrode and almost it does not take place on the
stainless steel electrode.
On the platinum electrode the reversible process
Mn7+ + e- ↔ Mn6+, is illustrated. This process takes place
at 450 mV for the oxidation and 300 mV for the reduction.
The process of anodic regeneration of Mn7+ on the platinum
electrode is promoted by the increase of:
- the concentration of KOH;
- the temperature;
- the concentration of Mn7+ (and Mn6+)
- the quinoxaline concentration.
The stainless steel electrode shows slight
polarizations or depolarisations of the process of oxygen
release so that it seems that the oxidation of Mn6+ to Mn7+,
is not possible. Nevertheless a final decision will be made
only after carrying out new investigations (preliminary
syntheses).
Figure 7. T = 40 °C, sol. KOH 10%, concentration of KMnO4: 1 – 0,
2 – 2·10-2 M, 3 – 3.8·10-2 M, 4 – 6·10-2 M, A – stainless steel electrode,
B – platinum electrode.
REFERENCES
1. Popa I.,
Dragos A., Taranu I., Stefanut M., Vaszilcsin C.,
Electrochemical
Syntessis Of Pyrazine-2,3-Dicarboxylic Acid,
Electrochemically kinetic studies on platinum electrode, Anniversary
Symposium „INCEMC – 10 years of existence”, Timişoara, 19-20 of
April 2007.
2. Dragos A., Popa I., Taranu I., Electrochemical Syntessis Of Pyrazine2,3-Dicarboxylic Acid, Electrochemically kinetic studies on platinum
electrode in quinoxaline presence, The Rumanian Academy Days,
Timişoara, 24-25 of May 2007.
3. Taranu I., Dragos A., Popa I., Electrochemical Syntessis Of Pyrazine2,3-Dicarboxylic Acid, Electrochemically kinetic studies on nickel
electrode, The Rumanian Academy Days, Timişoara, 24-25 of May 2007.
4.
Taranu I., Popa I., Dragos A., Vlatanescu N., Buzatu D.,
Electrochemical Syntessis Of Pyrazine-2,3-Dicarboxylic Acid, Cyclic
Voltammetry Studies On Copper Electrode, The Chemistry and Chemical
Engineering International Conference, 28 -30 of May 2008.
Figure 8. T = 40 °C, sol. KOH 10%, concentration of KMnO4 6·10-2 M,
concentration of Q: 1 – 0, 2 – 1.18·10-2, 3 – 2.04·10-2, 4 – 2.61·10-2 M,
A – stainless steel electrode, B – platinum electrode.
If on the stainless steel electrode (A) we cannot notice
any modification of the cyclic voltammograms obtained on
the platinum electrode, as an increase of the Q
concentration the 4 peaks observed modify by the
following:
45